1. Field of the Invention
This invention relates to an adjustable resistor of which resistance is finely adjustable, for example, during the period of or after having completed assembly of an electric circuit, and more particularly to the terminals of such a resistor.
2. Description of the Prior Art
FIG. 5 illustrates in section an adjustable resistor known in the prior art; FIG. 6 gives a perspective view of the same. In these drawings, reference character 1 designates a molded box-shaped case consisting of a bottom 2 and side walls 3, and the upper opening of the case 1 is indicated by reference character 3a. In the molded case 1, there are provided a resistor base 4 which has a resistor pattern thereon, and a slider mounted rotatably on the resistor base 4. The slider is provided with an engagement notch 5a for rotating it substantially at the center of the upper part and a slidable contact 5b as the lower part. There is further provided an eyelet 6 just fitted in a center through hole 4a of the resistor base 4. The upper flanged end of the eyelet 6 is forcedly placed on the inside surface of the slider 5. The lower end of the eyelet 6 extends through a side wall of the case 1 at a relatively lower level to the outside, and further downwards alongside the side wall and the undersurface of the bottom 2 to embrace the edge of the bottom, as indicated with reference character 6a.
The resistor base 4 has thereon a resistor pattern of which both ends 7 extend through the side wall of the molded case 1 at a relatively low level to the outside, and further downwards alongside the side wall and the undersurface of the bottom 2 to embrace the edge of the bottom, as in the case of terminal 6a.
The opening 3a of the molded case 1 is covered with a cover 8 of heatproofing film adhered or fused to the edges of the molded case 1.
Referring to FIGS. 7 and 8 illustrative of the process of soldering this adjustable resistor 9 to a printed circuit substrate 10, which has on the surface electrodes 11 of copper foil arranged in a predetermined pattern in opposed relation to the terminals 6a and 7 of the adjustable resistor 9.
The adjustable resistor 9 is attached provisionally to the printed circuit substrate 10 by means of an adhesive 12 with the terminals 6a and 7 opposed to the electrodes 11.
The printed circuit substrate 10 with the resistor 9 provisionally attached thereto is immersed in solder jet 13 to solder the terminals 6a and 7 to the electrodes 11 for completing electrical connection between them.
For preventing solder or flux from invading in the molded case 1 because, if it occurred, there would be danger of impairing the function of the resistor, the opening 3a of the molded case 1 is closed with a cover 8.
After the attachment of the adjustable resistor 9 to the printed circuit substrate 10, the cover 8 is removed by tearing off or breaking through. Then an adjuster is inserted to bring its lower end into engagement with the engagement notch 5a of the slider, and the slidable contact 5b is rotated by sliding on the resistor pattern to adjust it to a desired resistance value.
Now in the process of soldering the adjustable resistor 9 of the prior art to the printed circuit substrate 10, the problem is encountered that, as shown in FIG. 7, solder jet 13 flows in a curve in the vicinity of the area of electrodes 11 of the printed circuit base 10 and the terminals 6a and 7 of the adjustable resistors. However, it is difficult to make the solder flow into the area, because solder jet 13 has a large interfacial tension such that it forms a spherical surface for a small interface, for example, at the corner in this case.
Notably, referring to FIGS. 9 and 10, in the process where a plurality of adjustable resistors 9 are attached to a printed circuit substrate (10), at short intervals with the terminals 6a and 7 interopposed, the narrower the gap "A" between the opposed side walls of the adjacent resistors 9, the more difficult the penetration of solder jet 13 into the gap "A" becomes. This results in the defect that the solder jet cannot enter the areas of the terminals 6a and 7 in the gap A. Accordingly, it is impossible to make concurrent attachment of a plurality of such adjustable resistors to a printed circuit substrate 10. Thus has arisen a great problem in the way of high density attachment.
It therefore is an object of the invention to solve the above-stated problem encountered in the prior art and to provide an adjustable resistor permitting concurrent attachment of resistors at a high density to printed circuit substrate by soldering the terminals to the electrodes of a printed circuit substrate.
The object is attainable by the construction according to the invention that an adjustable resistor comprises a slider rotatably mounted on a resistor base, a case containing the slider and the resistor base, and terminals connecting to the slider and the resistor base, respectively, and extending through the bottom of the case to the outside. Fine adjustment of the resistance value is accomplishable by rotating the slider on the resistor base, wherein the terminals have exposed parts each consisting of a stem extending alongside the side wall of the case from the bottom close to the upper edge and branch parts extending laterally close to the side edge.
The above-described construction according to the invention enables foil-proof soldering of the terminals to the electrodes. Each terminal has an extension alongside the sidewall of the case consisting of an upright stem and mono- or bilateral branches for inducing solder flow towards the area where the terminals and electrodes are to be soldered together without fail.